Gain control



April 13, 17943. E. MORITZ, JR

GAIN CONTROL Filed Jan. 18, 1940 3 Sheets-Sheet 1 INVENTOR f/l/'IQ/llnbr @XM ATTORNEY April 13, 1943. B. E. MORITZ, JR 2,316,354

GAIN CONTROL Filed Jan. 1s, 19404 a sheets-sheet 2 INVENTOR Bur E. ./l/loritmiqr wsu/0109.135' B-Y Y l l u a @Ml/MM@ ATTORNEY April 13, 1943. B. E. MORITZ, JR 2,316,354

GAIN CONTROL Filed Jan, 18, 1940 3 Sheets-Sheet 3 Patented Apr. 13, 1943 GAIN coN'raoL Burt E. Moritz,- Jr., Houston, Tex., assignor to Stanolind Oil and Gas Company, Tulsa, Okla., a corporation of Delaware 1 I Application January 18, 1940, Serial No. 314,503

8 Claims.

This invention relates to control devices incorporated in thermionic vacuum tube amplifiers.

This .particular type of controlled amplifier is highly advantageous in the field of seismic geophysical prospecting, but the principles involved are applicable to widely different fields. It will be described in connection with the application to seismic prospecting.

Investigators working in this field' haverecog nized the fact that the amplitudes of the series of related seismic waves received after the detonation of a charge of explosive at a string ofdetectors placed in the neighborhood of a shotpoint will, in general, decrease rapidly with time. In order to compute the thicknessA of the weathered layer and for other purposes, it is necessary to obtainthe first breaks of seismic energy at each instrument, while in order to detect all of the reflected waves it is necessaryto i record on the same record the later, weaker waves. This imposes very strict requirements upon the types of amplifiers which are used in this connection because the maximum amplification must not only vary according to the spaced relationship of the seismometers with respect to the 'shot point but the amplifier associated with any seismometer, or group ofseismometers, must have a variable gain which in general increases with time after the detonation of the explosive.

In order to satisfy the latter requirement several methods of varying the gain of the seismic amplifiers With time have been proposed. Among these are the use of a variable attenuator which is manually varied with time at the will of the observer, and the use of a control voltage which is applied to one or more grids of variable gain amplifier tubes, either in accordance with a predetermined function of time or in inverse relationship with the amplitude of the recorded signal.l All three of these systems have obvious difficulties. The manually operated attenuator system depends for effectiveness upon the skill of a trained observer, and is comparatively useless in unskilled hands. .The use of a control voltage which varies as a fixed and invariable function of time is subject to the criticism that it will change the gain of the amplifier according to a predetermined schedule which may not entirely match the variation of the signal amplitude at any one location in the field. The third system which is known loosely as the automatic volume control system is subject to the inherent disadvantage that the amplitude of the control voltage which is applied to correct any variation in signal from a predetermined level is in reality determined by the average signal intensity over a certain length of time so that, if a strong signal is received, it will cause the gain of the amplifier system to be de- 'creased for a considerable period of time so that any weak signal following closely after the'strong signal will be recorded with a very low gain and may .be entirely overlooked in inspecting the records.

This invention discloses an amplifying systemv which is not subject to any of the difliculties mentioned immediately above. Accordingly, it is an object of this invention to provide a method of, and devices for, gain control in which the gain of the amplifier is not adjusted manually but automatically, in which the variation of the gain is not a fixed function of the time after the detonation of the explosive, and in which weaker signals following immediately after strong signals will not be recorded with a gain which has been decreased due to the presence of the strong signal. In other words it is an object of this invention to provide a control system for varying the gain of an amplifier by which the gain is automatically increased whenever the output signal level falls below a fixed predetermined value, but in which the gain is not subsequently decreased during the recording period.

It is a further object of this invention to furnish means whereby the operation of the total control is fully automatic after it has been initiated.

A further object is to furnish means whereby this control can be made entirely automatic in that the first response of the seismometer or the firing impulse of the plaster will initiate the system. v

Another object of this invention is to provide anamplifying circuit in which the gain of the amplifier may be kept at a predetermined low initial level for a predetermined period of time, after which the gain of the amplifier will increase automatically whenever the output signal level falls below a predetermined amount, but will not decrease below any previous value throughout the recording period.

A further object is to provide a control system such as discussed immediately above, in which the maximum gain of the amplifier or the rate` which form part of the specification and are to be read in conjunction .Jith it. In these drawings, like or corresponding reference numerals in different figures refer to the same or corresponding parts.

Figure 1 shows in block diagrammatic form the general arrangement of the interdepending parts of the apparatus forming a complete recording system for seismic waves and illustrates the action of -the control apparatus.

Figure 2 shows a schematic diagram of a complete amplifier channel constructed in accordance with the invention.

Figure 3 shows a schematic view of another amplifier of the same general type as shown in Figure 2 but with certain important additions and modifications which have proved advantageous in actual operations.

In Figure 1 there is shown in diagrammatic form the essential elements cooperating to form my invention and also a seismometer anda re-' corder to complete the entire recording system. Signals generated by seismic Waves impinging on the seismometer are conducted to a variable gain section of an amplifier and then to a fixed gain section of the same amplifier where they receive their final amplification and are recorded in the recorder.

From the fixed gain section of the amplifier a signal proportional to the recording signal is l across the charged condenser is passed through a filter and applied to control the gain in the variable gain section of the amplifier.

Initially the condenser is charged up to a certain predetermined voltage, which voltage establishes the initial gain of the variable gain amplifier section. For a period after the first signals from the seismometer are received, the discharging system for the condenser is inoperative and during this time the amplifier is maintained at the initial minimum gain..

This time is determined by the setting of the time delay relay which is actuated either by the output from the amplifier, or by-the time break impulse and which after a certain predetermined time permits the discharge system to become opof the fixed gain .amplifier and is high when i;

strong signals are received and low when weak signals are being recorded. Accordingly the con'- denser will partially discharge whenever the output of the amplier falls below a certain predetermined value, at a rate dependent upon the decrease in signal level below this predetermined value.l The voltage across the condenser is applied -as already mentionedto the gain control grids of the variable gai-n section in such a manner that as the condenser voltage decreases, the gain increases. There is no provision for recharging the condenser during the recording period. As a consequence of this the gain of the variable gain section of the amplifier will never decrease during the recording period but will increase at a variable rate depending upo'n the magnitude of the incoming signals. This is the main principle upon which this invention is founded and because of this and for lack of a grid 32 is connected to the cathode.

better term the type of 'control employed is called 75 a unidirectional control. uting to the control system proper as distinguished from the amplifier system'proper, are enclosed in broken lines in Figure 1, for clarity in illustration. s

It may be seen from the above description that the basic object of this invention is accomplished by any type of Vcircuit in which the elements operate in accordance with the functions shown in Figure 1. The variation in gain is entirely automatic and is not dependent upon the attention of the observer. On the other hand the fact that the change in gain is always in the same direction shows that there can be no suppression of a weak signal following a strong signal. At the very worst a weak signal following immediately after a strong signal will be recorded'in its true relative amplitude, while if the weak signal follows at some later time it will ybe amplified to a greater degree and will appear larger than normal in relation to the strong wave. Thus the tendency of the automatic volume control type of amplifier to over-control after the passage of a strong signal is eliminated, a point which has proved in practice to be highly advantageous.

Figure 2 shows one type of amplifier built in accordance with the principles of this invention. In Figure 2 the seismometer and recorder have been eliminated but the points at which they are connected with the amplifier are shown respectively avs I and 2, and 3 and 4. In this figure a seismometer or group of seismometers arranged in any of the methods well known to the art, are connected to the primary of input transformer II. The secondary of this transformer is connected between the control grid of a pentode type tube I2 and ground. This tube is operated with a cathode bias circuit composed of a resistance I3 and a by-pass condenser I4, and with an indirectly heated cathode. Howevcr, a directly heated cathode and agrid biasbattery could be used in this circuit if desired. A battery or other source of heating current is connected to the heater I5 of this tube, as well as the heaters of all the other tubes, but as there is no novelty in this feature the battery and connecting wires had been eliminated for the sake of simplicity. The screen grid I6 of this tubelis biased positively by a battery I 'I which is preferably connected to the screen grid I6 through a simple filter 'composed of a series resistance I8 and a by-pass Icon- `denser I9. Potential for the plate 20 of tube I2 is furnished by a battery 2| through the plate resistance 22. In this particular circuit the gain control voltage. is applied to the suppressor grid 23 in a manner which will be subsequently described. In this figure tube I2 and the adjoining coupling circuits yconstitute the variable gain sec- -tion of the amplifier.

The variable gain section of the amplifier is shown resistance-capacity coupled to the fixed gain section of the amplifier by means of a coupling condenser 24 anda grid resistor 25. The pentcde tube4 26 and the circuits associated with it form a normal resistance-capacity coupled amplifier section. The cathode bias circuit is composed of resistance 21 and by-pass condenser 28. The screen grid 29 is biased by battery I'I preferably through a filter composed' of a 'resistance 30 and a by-pass condenser 3|. The suppressor The plate 33 is resistance-capacity coupled to the succeed- 'ing tube by plate resistance 3,4, coupling condenser 35, the grid resistor 36. The plate 33 is supplied with potential by battery 2|. Trlode The elements contribvalue of the recorded signal.

tube 31 isthe output tube of the xed gain section.4 It is shown with a cathode bias circuit composed of resistance 38 and by-pass condenser 39, and with the plate 40 coupled through the primary of transformer 4| to battery 2|. The secondary of the output transformer 2| is connected to a. recording element of the recorder. Tubes 26 and 31 and the associated circuit elements form the fixed gain section of the amplifier.v

Normally the plate signal of triode 31 is insufiicient to operate the rectifying circuit in the unidirectional control. This signal is accordingly amplified by coupling it through condenser 42 and grid resistor 43 to the grid 44 of another triode 45, which may suitably be used with a cathode biasing circuit 46 and by-pass condenser 41. The plate 48 of this triode is coupled to transformer 49 which in turn feeds the full wave rectifier circuit. This last is composed of double diode 50, the plates of which are connected to the opposite ends of the secondary of transformer 49'and the cathode of kwhich is connected through resistance 5| tothe center tap of the secondary of transformer l49. A by-pass condenser 52 is connected across resista-nce 5| to filter out the alternating current components of the rectified voltage. By means of this circuit a direct current isproduced across resistance 5|,which varies in accordance with the average This voltage is used to control the. discharge circuit and the gain control condenser in the following manner.

.Triode 53 is operated as a 'variable resistance.

The rectified voltage across resistance 5| is applied between cathode 54 and grid 55 of this tube so that any rectified signal will produce a negative bias on the grid with respect to the cathode. The plate 56 of the triode is grounded. It is well known in the art that` the effective resistance between the cathode and plate of a triode depends upon the grid bias and can be varied over an extremely wide range (for example from about 10,000 ohms to several megohms) as the bias is varied from zero to a high negative value. The effective resistance betweenvcathode 54 and plate 56 of triode 53 will vary in accordance with the rectified signal appearing across resistance 5| and will be high when this signal is large and low when this signal disappears.

`During the normal operation of the entire amplifier arm 51 of the relay indicated generally as 58, will make contact with contact 59 so that condenser 60, one side of which is grounded, will be connected through the effective resistance of tube 53 to the ground and will accordinglyy discharge at a rate governed by the capacity of condenser 60 and the resistance of tube 53. The potential across condenser- 60 is applied uthrough a filter composed of a series resistance 6| and a shunt condenser 62 to the suppressor grid 23 of variable gain vpentode |2.' Thegain of this tube will vary inversely with .the bias on the suppressor grid 23.

Initially arrn 51 of relay 58 is connected to contact 63, so that the potential of the gain control battery 64 is applied to charge condenser 60 negatively, with respect to the ground. This negative potential is made sufficiently high so that the initial gain of pentode |2 (which. is preferably of the remote cut-off type) is low enough so that the iirstfsignals arriving from the seismometer will'be amplified to the normal recording amplitude by the time they pass through the secondary of output transformer 4|.

In order to actuate the relay 58 the plate signals on tube 31 are applied through coupling condenser 65 and rectifier 66, which is preferably a simple copper oxide rectifier, to the actuating coil 61 of relay 58. By this means a rectl` fied current is applied to this coil 61 which, after the passage of the first few cycles of the incoming signal will be suflicient to throw arm 51 from its initial position in contact with contact 63 to its nal position in contact with contact 59. Until this' relay operates, the gain of the first tube is held in its initial minimum value and all signals which pass through the amplifier will be amplified to the same amount.

Relay 58 is preferably of the locking type so that once arm 51 is thrown over against the contact 59 the relay arm will not swing back during the recording period but must be reset after the end of the recording period. The circuit constants associated with the relay are so chosen that after a certain predetermined time the relay Will operate. After this time, as discussed above, the initial high negative charge across vcondenser 60 will leak off at a controlled rate to ground through the effective resistance of the triode 53. Since the resistance of this triode will be high wheneverthe output signals are strong the effective resistance of tube 53 will also be condenser 60 cannot be recharged, the voltage across it can never exceed lany previous value andthe gain of the variable gain section of the amplifier can never decrease during one recording period,

Figure 3 shows the schematic diagram ofanother amplifier built according to the invention which has been modified to incorporate a number of features found to be very desirable in field operation. Incoming signals from the seismometer or seismometers are applied as before,

to the input of transformer across terminals and 2'. In this ease an especially advantageous type of tube, a pentagrid ccnverter is used in the variable gain amplifier section. 'I'his tube i 66, which may preferably be of the 6A8G type. or one with similar electrical characteristics, is operated with the signal applied between the control grid and the mixer grid as will be subsequently described. Preferably this tube is operated with a cathode bias circuit which is composed of by-pass condenser |4 and resistance 69. 'Ihis resistance is tapped so that it can be used as a variable sensitivity control. When switch 10 is in the lowest position the tube is operated with maximum negative bias and has low gain, while when the switch is in the upper position the tube is 'operating at high maximum gain. 'Potential on the screen grid 1| of this tube is obtained from battery 12 through a voltage divider composed of resistances 13 and 14. Resistance 14 is by-passed by condenser 15 for adequate filtering. In a multi-channel amplifier the resistances 14 of the various channels will be adjusted-sothat all channels will have the same gain for identical biases on the other grids of the pentagrid converter tubes. lIhis feature is iesirable to compensate for the non-uniformity 3f the tubes 'which in practice is always exoerienced.

Plate 16 of this variable gain section of the amplifier is resistance-capacity coupled through plate resistance 22, coupling condenser 24, and grid resistance to the first tube of the fixed gain section of the amplifier. A plate filter circuit composed of resistance 11 and by-pass condenser 18 is employed to prevent any possibility of mutual coupling of any stages through the impedance of battery 12.

The signal across grid resistor 25 is impressed on grid 32 of the pentode 26. This tube is operated as a conventional cathode-biased resistance coupled ampiler stage. The cathode biascircuit is composed of resistance 21 and condenser 28. The screen grid of this tube is supplied with potential from battery I1 through a suitable lter composed of resistance and condenser 3|. Ihe plate 33 of this tube 26 is connected through plate resistance 34 and lter resistance 19 to battery 12 and, as in the previous stage, a by-pass condenser 80 is employed in conjunction with filter resistance 19. The output of this tube 26 is coupled through coupling condenser 35 and series resistance 8| to a band rejection lter indicated generally as 82. Many types of band rejection filters are known in the art which could be suitably employed here. One particular example is described simply by way of illustration and not by way of limitation. This is a pi-section iilter with a shunt arm composed of inductance 83 and condenser 84 across which is placed resistance 85, a series arm made up of `resistance 86, and a second shunt arm made up of inductance 81 and condenser 88. This lter section may be tuned to any desirable frequency, for example, it may be tuned to reject 10 cycles, which will minimize the major ground roll components of the incoming signal. It also serves another advantageous purpose which will be later described. Normally several filters having different characteristics will be made up and used interchangeably to vary the overall frequency response curve of the amnected to gain control battery 64.

pliiler which in field practice is found to be quite useful. The output of this filter is coupled through condenser 89 to tapped resistance 90, which with grid resistance 36 forms an adjustable volume control. A shunt condenser 92 is placed across grid resistance 36 so that the high frequency components of the incoming signal such as wind noise, etc., are attenuated.

vOne-half of a double triode tube 93, for example a GFSG, is used as the output tube of the fixed gain section of the amplier. The plate 94 of the tube is connected to the primary of output transformer 4|, the other side of which is connected through a plate filter made up of a resistance 95 and by-pass condenser 96 to battery 12. The secondary of transformer 4| is connected to one recording element of the recorder.

The signal on the plate 94 of double triode 93 is also impressed upon the second triode grid through coupling condenser 42 and grid resistance43. 'Ihe two cathodes of this double triode tube 93 are suitably biased by biasing circuits conslsting of resistance 38 and by-pass condenser 39,

and resistance 46 and by-pass condenser 41, re-

|00 to battery 12. A transformer 49 with a center tapped secondary, is employed to impress the signal across resistance 98 upon the two diode plates of a double triode-pentode |0|, a double diode-triode, or the like. This tube forms the controlled discharge circuit and takes the place of triode 53 and the double diode 50 of Figure 2. I have found that a tube of the 6B? type used with the pentode connected as a triode, is especially advantageous.

The rectified output of the signal impressed across the secondary of transformer 49 appears across resistors |02, 5|, and |03 which are in series between the cathode |04 of tube |0| and the center tap on the secondary of transformer 49. Of this rectified signal the portion appearing across resistance |02 and 5| is used as the bias between cathode |04 and control grid |05 ol tube |0l. Condenser 52 is shunted across resistance 5| to by-pass the alternating current component of the rectified lcurrent which would otherwise appear across this resistance. By a suitable choice of resistors 5| and |03 it is possible to select; any arbitrary portion of the total rectied signal for use as a grid bias in tube |0|. The screen and suppressor grids of tube |0| are tied to the plate' and the combination is grounded.

The junction between resistance |02 and 5| is connected to one fixed contact |06 of a single pole double throw set of relay contacts in the time delay relay. The opposite fixed contact |01 is con- The movable contact |08 is connected to the non-grounded side of condenser which operates in this circuit in exactly the same method outlined in connection with Figure 2. Thus, until the relay coil 61 is energized, condenser 60 is maintained charged to the potential of gain control battery 64 through contacts |01 and |08, and this potential is applied through the low pass iilter composed of resistance 6| and shunt condenser 62 to the oscillator and control grids of the variable gain tube 68, maintaining the gain of this tube at its initial low value.

After the relay coil 61 is energized and the time delay relay operates, contact |08 is connected to contact |06 throughout the remainder of the recording period,`and as before, condenser 60 is discharged through the effective resistance of the triode-connected section of tube |0I. Whenever the output signal is high the rectied voltage across resistances |02 and 5| will be large, the

negative bias of grid |05 with respect to cathode |04 will be high, and the effective resistance between the cathode and plate of that tube will be large so that condenser 60 will discharge at a very low rate. A decrease in rectied voltage will accelerate the discharge of condenser 60. Resistance 02 in this circuit is in series with the effective tube resistance and is inserted so that the discharge of condenser 60 will not be too rapid when the eiective resistance of tube |0| drops to its minimum value. @Accordingly the operation of tube |0| can be considered as that of a noiseless switch grounding one end of resistance Il resistance |02 were .not resent, th..I` c" of gain of tube 68 would be so rapid on oo' .sions .that a spurious signal of low ,frequency would be generated in this tube and recorded. Hence resistance |02 helps prevent such ciani'cs. Tlv recording of such signals is also obvalcd by iii use of the low frequency band rejection filtri' 92. Low frequency oscillation is also prevented by this circuit.

asiaasa In practice it is found that the effective resistance of tube IOI of the 6B? type varies from about 20 megohms to about 30,000 ohms for approximately one volt change in grid bias. As discussed above, the point at which this change takes place can be adjusted by varying the relative Values of the two resistances I and |03. 'Ihis ratio is adjusted by experiment to give the correct overall record level.

Figure 3 illustrates another type of actuating circuit for the relay coil 61 which differs from that shown in Figure 2 in that the relay is here actuated a certain time after the receipt of the time break impulse from the blaster used to detonate the explosive. In this circuit the primary of transformer |09 is connected to this'time break circuit across contacts 5 and 6 sothat the voltage generated at the instant the cap wire fuses is impressed across this circuit.

The secondary of transformer |09 is connected cathode and grid of a gaseous discharge tube (Thyratron or grid-glow tube) III. The plate of this tube is connected through a push-to-open switch I I2, through the energizing coil of the time delay relay and through plate battery H3 to the cathode of tube II I. The bias imposed on the grid of tube II by the bias battery I I0 is adjusted so that, upon receipt of the time break impulse, tube` III is fired, after which a steady current ows through coil 61 until push button II2' is operated. The magnetic core enclosed by coil 61 is fitted with a heavy copper ring so thata definite predetermined time elapses between the energizing of this coil and the operation of the relay contacts. It is understood of course, that many other types of suitable time delay relay actuating circuits could be used instead of the type described. By use of the initiating circuit shown, or a similar one, the variable gain tube 68 is maintained at its initial low value for a deiinite predetermined time after the reception of the time break impulse at the recording truck, after which this gain is controlled by the unidirectional control circuit described. y

This arrangement has the advantage that there is no need to operate the gain control circuit during the rst few hundredths of a second-after the initial receipt of seismic waves, so-that the control need be effectivev only over a greatly decreased range. However, during these rst few hundredths of a second, waves of greatly varying magnitude are encountered, some of which are often so large in relation to the iirst wave received that if this first Wave is recorded with perceptible amplitude (which is always necessary for certain One before the control takes hold, so that the waves can be recorded on the limited width of the film allotted each trace.

Referring to Figure 3, a connectionis shown from one side of the primary of the output transformer 4I leading to a blocking condenser I I4, through a full wave rectifier, preferably of the copper-oxide type shown generally as I I5, through contacts II 9 .and |20 `of relay 58, through resistance IIS,l and through a filter circuit composed of resistance II'I and by-pass condenser Y |I8 to ground. The voltage across the primary transformer 4I, so arranged that there is eiectively no recticatonuntil the signal amplitude increases above a certain predetermined value, after which the rectiiication increases as the signalintensity increases above this level. By

use of this circuit, the peaks of undesirably large amplitude waves are reduced during the period of output transformer 4I is thus impressed effectively across resistance IIS and copper oxide rectifier II5, in series. The effective resistance of this copper oxide rectifier varies roughly inversely with the applied potential across it, so that until the drop across the rectifier I I5 exceeds a certain value there will be very little rectification.. After this point is passed, the resistance of the rectifier will decrease, thus lowering the value of the shunt impedance applied across the primary of output transformer 4I and by-passing more and more of the strong signals. Since resistance IIS and rectifier I|5 forma voltage dilvider, it is possible by selecting the impedance of these two elements to cause the by-passing action to occur at any arbitrarily chosen value of output signal strength.

It is desirable to eliminate the rectifier froml this shunt circuit, in fact it is desirableto eliminate the shunt circuit entirely after the control circuit has been placed in operation, so that there will be no distortion on the peaks of the recorded waves. It is quite important that minimum distortion occur in order that correlation between different records will be possible. In order to accomplish this. the relay contacts II9 and I 20 of the time delay relay arel so arranged that as soon as the control circuit is actuated, these contacts open and remove the shunt across the primary of output transformer 4 I. It is understood that there are numerous other types of volume limiters which could bei used instead of the type shown, and that this intention is not to be nmitea by the use of any particular type of volume limiter circuit.

As has been previously stated, the circuits shown are merely for the purpose of illustration rather than limitation. The invention is best defined and limited by the appended claims.

I claim:

1. In apparatus for seismic surveying comprising a seismometer, an amplifier associated with said seismometer, said ampliiier including a vari` able gain section, and a recorder actuated by said seismometer through said amplifier, he improvement which comprises gain control operative only to increase the gain of said variable gain section of said amplifier, and means responsive to signals passing through said variable gain section of said amplifier for continuously controlling the rate of increase in gain of said gain control.

2. In apparatus for seismic surveying comprising a seismometer, an amplifier associated with said seismometer, said amplifier including a variable gain section, and a recorder actuated by said seismometer through said amplifier, the improvement which comprises a rectifier for a signal which has passed through said variable gain section, and means actuated by said rectifier controlling the gain of said variable gain section of said amplifier whereby the gain of said variable gain section is increased whenever the amplied output of said amplifier decreases below a predetermined valuevand is maintained substantially constant whenever the output of said ampliiier considerably exceeds said predetermined value.

3,. Apparatus for seismic surveying comprising a seismometer, an ampliiier associated with said seismometer, said amplifier including a variable gain section, a recorder actuated by said seismometer through said ampliiier, a rectifier for rectifying a signal which has passed through said variable gain section of said ampliiier, and means responsive to the output of said rectiiier and acting on said variable gain section of said ampliiier to increase the gain of said variable gain section ofv said amplifier in response to decreases in the strength of said rectiiied signals while maintaining said gain substantially constant in response to increases in the strength of said rectified signals.

4. Apparatus for seismic surveying comprising a seismometer, an ampliiier associated with said seismometer, saidampliiier including a variable gain section, a recorder actuated Aby said seismometer through said amplifier, means responsive to the amplitude of signals which have passed through said variable gain section of said amplifier for varying the gain of said variable gain section of said amplifier solely in the direction of increasing gain at a decreasing rate as saidv amplitude of said signals increases, and means for initiating the action of said last-mentioned means at an interval of time after the iirst arrival of said signals at said amplifier.

' 5. Apparatus for seismic surveying comprising means-for generating seismic Waves, a seismom-l eter, an amplifierl associated with said seismometer, said amplified including a variable gain section, a recorder actuated by said seismometer through said amplifier, means for changing theA gain of said variable gain section of said ampliiier solely in the direction of increasing gain, means responsive to the average amplitude of the signals passing the output of said amplifier for varying the rate of change'of said variation in gain, and means responsive to' the operation of said iirstmentioned means and operative a predetermined time after the operation of said first-mentioned means for initiating the action of said last-mentioned means at a time interval after the arrival of the rst of said signals at said amplifier.

6. In a method of seismic surveying comprising receiving seismic Waves and converting them into corresponding electrical signals, variably aniplifying said electrical signals, and recording said amplied signals, the improvement which comprises automatically increasing said variable amplification from time toA time during at least a part of the seismic surveying operation in response to decreases in the strength of said Variably ampliiied signals while maintaining the change in said amplication substantially zero during increases in the strength of said variably ampliiied signals.

'7. In a method of seismic surveying comprising receiving seismic waves, and converting them into corresponding electrical signals, amplifying said electrical signals, and recording said amplified signals, the improvement which comprises maintaining the amplication constant during the first part of a seismic surveying operation and thereafter continuously and automatically increasing said amplification in response to the strength of said amplified signals at a rate varying approximate1yinverse1y with the strength of said ampiied signals.

8. A method of amplifying a series of seismic detector signals which comprises amplifying signals by a vacuum tube amplifier circuit having a portion of such character that application of a decreasing control bias thereto increases the amplication gain, applying a progressively decreasing bias' to said portion of the circuit whereby gain progressively increases with time, and arresting the decrease in bias upon appearance of amplified signal energy above a predetermined amplitude corresponding to a signal amplitude level above which signals cannot usefully be recorded.

BURT E. MORITZ, JUNIOR. 

